1. Field of the Invention
This invention relates to radio frequency power amplifiers and, more particularly, to a method to be used within a radio frequency multi-stage power amplifier to increase linearity and efficiency of the amplifier.
2. Discussion
Multi-stage radio frequency power amplifiers are used in a wide variety of communications and other electronic applications. These amplifiers are made up of two or more cascaded amplifier stages, each of which increases the level of the signal applied to the input of that stage by an amount known as the stage gain. The multi-stage amplifier output is therefore a replica of the input signal increased by the product of the stage gains. Ideally the input to output transfer of each stage would be linear; a perfect replica of the input signal, increased in amplitude, would appear at the amplifier output. Practical amplifiers, however, have a degree of non-linearity in their transfer characteristic. This non-linearity results in the distortion of the output signal so that it is no longer a perfect amplified replica of the input. One manifestation of this distortion is the creation of spurious signal components, known as intermodulation products, at frequencies which did not exist at the original input. These intermodulation components have a deleterious effect on the performance of the system employing the power amplifiers.
Two principal approaches have previously been used to increase linearity and thereby reduce the generation of intermodulation products. The first is the class A operation of each of the amplifier stages. This class of operation features the highest linearity but is often impractical because of its very poor efficiency and high thermal dissipation characteristics. The second conventional approach is the employment of separate linearizer circuits. Feed-forward, feed-back, and pre-distortion linearizers are conventionally utilized for this application. The separate linearizer approach suffers from several significant disadvantages. It significantly increases the cost of the amplifier and requires complex tuning and alignment to track the inherent variations of the amplifier over time and environmental changes. Additionally, the degree of linearization possible with a multi-stage amplifier is limited by the non-linear combination of the distortions generated by each stage.
The present invention overcomes the inherent disadvantages of the conventional approaches. The amplifier is designed so that the intermodulation components generated in each pair of cascaded stages are of opposite phase. In this manner the intermodulation components will combine subtractively thereby reducing the component level over the range of operation of the amplifier.